Our review

A guide to designing event models and writing Event Models in EventModeler format, following a 7-step methodology.

Strengths

Structured approach with clear steps and standardized format
Integrates modeling of commands, events, and views
Includes an append-only event stream for traceability
Defines slices as deliverable work units

Limitations

Requires good prior understanding of domain-driven design
ASCII wireframes may be limiting
Does not cover detailed technical implementation

When to use it

Use during the design phase of an event-driven system to structure business conversations and align stakeholders.

When not to use it

Avoid for very simple projects without event complexity or when the team is not familiar with event modeling concepts.

Examples

Shopping Cart Event Model

Create an event model for an e-commerce shopping cart system using the /event-model-plan methodology. Include slices for adding items, updating quantities, and placing an order. Provide the emlang blocks and event stream.

User Registration Event Stream

Design an event model for user registration with event stream entries. Include commands like RegisterUser, events like UserRegistered, and views like UserProfile. Show the frontmatter and traceability.

/event-model-plan — Event Modeling & Event Model Format

You are helping design event models and write Event Models for EventModeler. Follow the Event Modeling methodology and the Event Model format defined in docs/product-spec.md.

Event Modeling Workshop (7 Steps)

Brainstorm — Capture all domain events (past-tense: UserRegistered, OrderPlaced)
Plot — Arrange events chronologically on a timeline (left to right)
Storyboard — Add wireframes above the timeline showing what users see
Inputs — Add Commands linking user actions to Events (RegisterUser → UserRegistered)
Outputs — Add Views showing what data users see after events (UserProfile, Dashboard)
Organize — Define swimlanes for actors and bounded contexts
Elaborate — Define Given/When/Then scenarios for complex business logic

Slice Definition

A slice is a vertical unit of work: Trigger → Command → Event → View.

Use emlang notation:

slices:
  SliceName:
    steps:
      - t: Actor/TriggerScreen       # Wireframe/trigger
      - c: CommandName                # Command
        props:
          field: type
      - e: Context/EventName          # Event
        props:
          field: type
      - v: ViewName                   # View/read model
    tests:
      HappyPath:
        when:
          - c: CommandName
            props:
              field: value
        then:
          - e: Context/EventName
      ErrorCase:
        given:
          - e: Context/SomeEvent
        when:
          - c: CommandName
        then:
          - x: ErrorName

Event Model Format

Structure

---
title: "Feature Name"
status: draft | modeling | refined | approved
domain: "Bounded Context"
version: 1
---

# Feature Name
## Overview
## Key Ideas
## Slices
### Slice: Name
(emlang block)
## Scenarios
## Data Flows
## Dependencies
## Sources
<!-- event-stream -->
## Event Stream
(eventstream blocks)

Event Stream

Append-only log at the bottom of Event Model files. Each event is a fenced eventstream block:

seq: 1
ts: "2026-02-21T10:00:00Z"
type: EventModelCreated
actor: user@example.com
data:
  title: "Feature Name"
  status: draft

Key Rules

Emlang blocks: one per slice, under ### Slice: heading
Event stream: append-only, never modify existing blocks
Frontmatter status tracks Event Model lifecycle: draft → modeling → refined → approved
Wireframes are ASCII art (plain text, no images)

Guidance

When helping design event models:

Start with domain events — what happens in the business?
Group events into slices — each slice is a deliverable unit
Define commands — what action triggers each event?
Add views — what does the user see after each event?
Write scenarios — Given (preconditions) / When (command) / Then (outcomes)
Check completeness — every view field must trace back to an event field, which traces to a command field

Event Modeling & Event Model Format

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